There is a huge interest in using furandicarboxylic acids as a monomer to produce polymers. So far, this has been completely focused on 2,5 furandicarboxylic acid (2,5-FDCA), the monomer for the production of PEF (poly(ethylene 2,5-furandicarboxylate)) and other polymers. This resin has been considered as one of the potential substitutes for PET (polyethylene terephthalate) because it offers similar properties and can be produced from renewable sources. The potential of 2,4-furandicarboxylic acid up to now has been overlooked by researchers, only a few production routes have been described.
The article “Reaction of Vanillin and its Derived Compounds. XIV. 2,4-Furanedicarboxylic Acid from Vanillin” (Pearl et al.) mentions Feist et al., where the production of 2,4-FDCA from methyl coumalate was explained. However, that precursor is much more expensive than furoic acids. The use of vanillin is also analyzed in the paper, but the yield was less than 3%. The reaction medium for the synthesis from vanillin is very complex.
Hachihama et al. describes, in “Syntheses of Polyesters containing Furan Ring”, the synthesis of 2,4-FDCA through a four-step process, starting from 2 moles of malic acid, via methylcoumalate, in an overall yield less than 15% and without valuable byproduct formation. The approach starting from malic acid does not only result in a very low yield but also comprises steps that require stoichiometric reagents like HBr and several complex and different solvent systems like sulphuric acid, methanol and chloroform.
The article also describes one example of a 2,4-FDCA based polyester. The melting point of that polymer is lower than that of 2,5-FDCA based ones, but the data supplied are excessively limited to draw any conclusion about the potential properties of that resin.
The Italian article “Ricerche sulla migrazione del gruppo carbossilico nei sistemi eterociclici—Nota I. Sulfa preparazione dell'acido 2-5-furandicarbossilico da acido furoico” (Andrisano et al.) describes the synthesis of 2,5-FDCA through the disproportionation route.
Andrisano et al reported that the potassium salt of furoic acid when heated up to 250-300° C. in nitrogen atmosphere, undergoes decarboxylation to furan with simultaneous carboxylation at the 5-position to dipotassium 2,5 furandicarboxylate.
This disproportionation reaction subsequently has been overlooked in the recent attention for developing renewable routes to 2,5-FDCA. That this route should have potential is nonetheless clear from the disproportionation or thermal rearrangement of alkaline salts of aromatic carboxylates to symmetrical aromatic dicarboxylates. This reaction is known as the Henkel reaction (also called Raecke process) and is usually carried out in the presence of cadmium or other metal salts. As mentioned before, this process yields symmetrical aromatic dicarboxylates (which can be acidified to yield dicarboxylic acids). Thus, it is highly unexpected that the disproportionation of K-furoate yields an asymmetrical compound like the 2,4-FDCA.
Therefore, considering the problems of low yield, excessive number of process steps, presence of undesired byproducts and the cost of the reagents, the objective of this invention is to provide a route to synthesize 2,4-FDCA in a 2-step process from cheap biomass (products derived from furfural), with elevated yield and absence of toxic byproducts. Up to now there has been little research on the properties of the 2,4-FDCA based polyester, and virtually no studies analyzing its impacts as comonomer in PEF or other polymers and products. The combination of those 2,5-FDCA and 2,4-FDCA monomers might generate a synergic effect such as that of ethylene terephthalate and ethylene isophthalate in the macromolecular structure and properties of PET such as crystallization and melting point.